This invention (method and apparatus) relates to carburizing ferrous articles, parts or workpieces and conceptually related processes such as carbonitriding. Carburizing may be defined as the introduction or application of additional carbon to the surface of a ferrous metal article with the object of increasing the carbon content of the surface, and to some limited depth, beneath the surface (the depth of substantive penetration of the carbon hereinafter called “case”) of the article. When the article is subsequently subjected to an additional heat treatment, the surface portion carburizes resulting in a substantially harder surface than the underlying virgin or “green” metal. This is known in the art as “case hardening.”
Carburizing is an old and developed process. There are a number of methodologies which have been used to carburize ferrous parts. Perhaps the earliest application is “box carburizing” where open charcoal pits were used. Bone meal was packed around the parts to provide a protective atmosphere when heated and to be the source of carbon. That process has evolved into “pack carburizing” where parts to be carburized are packed into a box with a carburizing compound, such as metal carbonates burned to a hardwood charcoal by the use of oil, tar and the like, packed thereabout. Carbon is formed on the surface of the steel by the decomposition of carbon monoxide (from the carburizing compound) into carbon and carbon dioxide. The carbon dioxide that is formed reacts immediately with the uncondensed carbon in the carburizing compound to produce fresh carbon monoxide. This process is repeated as long as there is enough carbon present to react with the excess of carbon dioxide and until the surface of the ferrous part is saturated. This “class” of carburizing is distinguished from the prior art to which this invention relates by its requirement for a solid carburizing compound “packed” about the workpiece. Another process which is used is liquid carburizing in which the steel or iron is placed in a molten salt bath that contains chemicals such as barium cyanide and the like required to produce a chafe comparable with one resulting from pack carburizing. The piece is placed in the bath for a predetermined length of time at elevated temperature such that the carbon diffuses into the surface of the metal. This “class” of carburizing is distinguished from the prior art to which this invention relates by its requirement for a liquid or salt bath into which the workpiece is submerged.
This invention generally relates to carburizing by “gas” in that a gas containing carbon is used as a gaseous medium to provide gas phase carbon atoms to iron to produce the face centered iron with carbon in the matrix as well as iron carbide (Fe3C) precipitate. Gas carburizing can be further divided into atmosphere gas carburizing and vacuum carburizing with vacuum ion carburizing as a separate species of vacuum carburizing.
Atmosphere gas carburizing is a well developed technology which has proven acceptable for most case hardening carburizing applications. In atmosphere gas carburizing, a hydrocarbon, typically natural gas (methane), propane or butane, is metered into an endothermic gas furnace atmosphere maintained at positive pressure (i.e., at “atmospheric” pressure) in an industrial furnace. By controlling dew point of the gas composition (endothermic gas and carburizing gas), most typically the CO/CO2 gas ratio (water gas shift reaction), the gas carbon potential is controlled. Typically, the gas carbon potential is set at or near the saturation of carbon in the iron solution and when carbon in the iron matrix and iron carbide (Fe3C) precipitates are formed throughout the surface, the gas carbon potential of the furnace atmosphere gas is changed to neutral (“equilibrium carburizing”) to allow the carbon to diffuse into the case. The diffusion can be controlled vis-a-vis gas composition and temperature. For example, it is possible with atmosphere gas carburizing to actually decarb (remove carbon from) the surface during diffusion to allow a harder part composition between part surface and “green” core (portion of virgin metal beneath surface not affected by carburizing) because the case depth is increasing during diffusion. Further, in atmosphere gas carburizing the carbon potential does not have to be set at saturation limits of the steel. Specifically the carbon potential can be set at lesser values to avoid a natural phenomenon occurring at saturation referred to herein as “carbide network”. That is, at saturation, the surface of the part comprises iron carbides closely packed as adjacent molecules of face centered carbon steel which can be viewed as linked together in a “carbide network.” When carbon diffusion occurs it is potentially possible that groups or clusters of the packed iron carbide molecules are not homogeneous throughout the case. Conventional metallurgical thinking in the trade is that over time and at high stress, the carbide network can function as a stress riser. Some metallurgists, however, do not share this opinion.
With atmosphere gas carburizing, the carbide network can be minimized by simply controlling the carbon potential to minimize the formation of the network in the first place. That is, if carburizing does not occur at saturation, the network is not likely to be formed. Atmosphere gas carburizing inherently produces metal oxides on the part surface because of the presence of oxygen in the atmosphere. For this reason, atmosphere gas carburizing is fundamentally different from vacuum gas carburizing which does not have oxygen. For this reason as well as other process considerations fundamentally arising from the use of vacuum and its affect on gas reactions, atmosphere gas carburizing is a carburizing class distinguishable from the prior art related to this invention. (Also, this invention is fundamentally different from vacuum carburizing prior art which uses oxygen. For example, see U.S. Pat. No. 4,386,973 to Kawka et al., issued Jun. 7, 1983, which discloses alcohol for use as a vacuum carburizing gas.) For closely controlled, high stress areas such as required in the aerospace industries and even for gear trains in vehicular applications, the presence of metal oxides which, among other things, produce stress risers and change part dimensions is not acceptable.
Vacuum carburizing avoids the formation of metal oxides because a hydrocarbon gas lacking oxygen is used and the furnace chamber is pumped down to a high vacuum (low pressure) to remove any oxygen that may be present. Vacuum furnaces are typically utilized for heat treating precision parts with strict case hardening specifications. In vacuum carburizing the furnace is pumped down to a vacuum and the part heated to a carburizing temperature under vacuum. The furnace is then backfilled (but still under vacuum) with a carburizing gas, typically propane or butane, which disassociates at the carburizing temperature to produce carbon molecules that go into solution with the iron and cause iron carbide as precipitate at the part surface. Because the carbon disassociation can not be controlled (the presence of vacuum and only the carburizing gas), vacuum carburizing proceeds at saturation limits (about 1.31% for carburizing plain carbon type steels at temperatures of 1700° F.) and produces the carbide network discussed above throughout the surface. (If the carburizing hydrocarbon gas is metered at less than carbon saturation potential, uneven carburizing occurs.) Often the saturation portion of the carburizing cycle is called the “boost” portion. Metering of the carburizing gas is stopped at this point and diffusion is allowed to proceed with or without a change in temperature. (If the vacuum pump continues to run after metering of the gas is stopped, the vacuum increases and the part is subjected to strictly diffusion. As used herein, “diffusion” covers both “boost diffusion” and diffusion at a fixed or set vacuum level.).
Again, it must be noted that many metallurgists believe that vacuum diffusion (Fickes Law, Harris Law) disassociates or breaks up the iron carbide network. Others believe that the network can exist as described above. Still others believe the iron carbide network is not undesirable. In any event, carbon in solution diffuses into the case when flow of carburizing gas stops and vacuum is maintained at the carburizing temperature. The cycle may be repeated until the proper depth of carbon penetration is obtained. The carburized part is subsequently either heated to a proper hardening temperature and transferred to a quench (either a gas quench at high bar or a liquid quench which can be either under vacuum or positive pressure in the furnace) or removed from the furnace and later reheated and case hardened. Typically, the vacuum furnace is a cold wall, water cooled pressure vessel heated by electric heating elements. Recent developments in this area have included the use of gas fired radiant tubes to replace the electric resistance heating elements (see U.S. Pat. No. 5,224,857 to Schultz et al., issued Jul. 6, 1993) and the development of gas fired, hot wall vacuum carburizing furnaces (see U.S. Pat. No. 5,228,850 to Hoetzl et al., issued Jul. 20, 1993 and U.S. Pat. No. 6,283,749 to Bernard et al., issued Sep. 4, 2001). Because of the hot wall configuration, the temperature for hardening applications may be limited in hot wall carburizing furnaces, but carburizing temperatures of 1700° F. are obtainable.
Some limitations present in conventional vacuum furnaces relate to the ability to uniformly carburize parts having convoluted surfaces such as certain types of gears or certain parts which may be tightly packed in work baskets hindering penetration of the carburizing gases. In such applications an ion carburizing furnace has been developed which develops a cold plasma that produces a glow about the workpiece (see, for example, U.S. Pat. No. 5,127,967 to Verhoff et al., issued Jul. 7, 1992). The carburizing gas is ionized in the glow discharge producing carbon. Typically, the parts are initially cleansed by ionizing a non-carbon bearing gas, such as hydrogen, in a “sputter clean” step. During carburizing, the glow discharge produces a uniform infusion of carbon over the irregular part surface. Like conventional vacuum carburizing, vacuum ion carburizing also has iron carbide network limitations since carbon diffuses into the surface until saturation. In addition a conventional vacuum furnace has to be fitted with a power supply and electrically insulated vis-a-vis its hearth so that gas ionization can proceed. This increases the expense of the furnace.
This invention relates to gas carburizing with vacuum and prior art classified as conventional vacuum carburizing or vacuum ion carburizing is pertinent to the present invention.
Typically, the carburizing gas used in vacuum carburizing is a lower order saturated aliphatic hydrocarbon such as propane or butane while gas atmosphere carburizing typically uses the simplest alkane, methane. However, other carburizing gases have been used in vacuum carburizing. U.S. Pat. No. 5,702,540 to Kubota, issued Dec. 30, 1997, is commercialized and discloses the use of acetylene as a carburizing gas. The gas has been promoted for its ability to achieve carbon diffusion in small holes having high L/D (hole length to hole diameter) ratios when compared to processes using the typical straight chain alkanes. Because of the sooting (carbon deposit) produced by acetylene, Kubota operates the furnace chamber at high vacuum (low pressure). The concept is to draw the carburizing gas out quickly so carbon cannot deposit while the abundant supply of carbon present in the acetylene gas is still sufficient to form a saturated iron carbide at the surface of the workpiece. Thus, Kubota uses an unsaturated aliphatic gas at high vacuum (low pressure) to prevent sooting. In contrast, U.S. Pat. No. 6,187,111 to Waka et al., issued Feb. 13, 2001, uses ethylene as a carburizing gas but at a lower vacuum level (higher pressure) than Kubota. According to Waka, if the vacuum is higher (low pressure) than the minimum, carburizing cannot occur and if the vacuum is lower (less pressure) than the maximum, carbon soot will form. U.S. Pat. No. 5,205,873 to Faure et al., issued Apr. 27, 1993, also discloses the use of the unsaturated aliphatic hydrocarbon, ethylene as a carburizing gas. However, Faure introduces hydrogen in a vacuum carburizing process. After preparing the work for carburizing, Faure backfills the furnace chamber with H2 to a pressure of about ½ atmosphere. Ethylene is then metered into the furnace while the chamber is pumped down to normal vacuum levels in the range of 7.5 to 75 torr. As the ethylene is pumped in, the H2 is pumped out. At the same time, cracking of ethylene produces H2, so some H2 is made up. However the H2 is being reduced during the process from a “high” of up to 60% at the beginning of the cycle to a “low” as little as 2% at cycle end. For reasons discussed below, while H2 is beneficial to the process, Faure is opposite to what is desired.
The literature has also recognized the trend to unsaturated higher order aliphatics. See, for example, the article “New Vacuum Carburizing Technology”, published in the February/March 2001 issue of Heat Treating Progress, at pages 57–60, which discusses ethylene and controlling tar deposits by introduction of hydrogen (and nitrogen) at high percentages. The article states that if hydrogen was maintained at greater than 60% of the gas composition in the furnace chamber, soot could be eliminated. For reasons discussed below, it is possible that repeatedly issues concerning the carburized case are present at the high percentages cited. The article “New Wrinkles in Low-Pressure Carburizing”, also published in the same issue of Heat Treating Progress, at pages 47–51, discusses acetylene in a plasma discharge application.
As is well known, the aliphatic hydrocarbons are divided into two groups, namely, the saturated aliphatics or alkanes or paraffins, and unsaturated aliphatics, which include both alkenes and alkynes. Alkenes are also referred to as olefins and alkynes are referred to as acetylenes. The alkenes or olefins have a carbon-carbon double bond and include compounds such as ethylene (or also called ethene) and is denoted chemically by the formula C2H4. Alkynes with the triple bond include gases or compounds like acetylene (also called ethyne) and is denoted by C2H2 or HC≡CH with triple bonds between the HC and CH molecules or carbon pairs. Alkanes include methane, ethane, propane, butane, pentane, hexane, heptane, octane, and nonane. All of the alkanes can be expressed as the formula of CnH(n*2+2) In regard to the alkenes and alkynes, a different but repeating process occurs for the hydrogen to carbon relationship. The family of alkenes is expressed by the relationship of CnH2n. Thus, in the case of ethylene C2H4, the number of “H” is strictly double that of the “C”. Also in the alkene family is propylene C3H6. The family of alkynes is expressed by the formula CnH2n−2. An example of this compound is acetylene C2H2.
In addition to the relationship between the hydrogen to carbon count, there is a special relationship regarding the number of bonds between the carbons. Alkanes have one bond between each C and as previously indicated, alkenes have two bonds and alkynes have three. In regard to the arrangement of the carbon hydrogens to each other, the aliphatics are characterized as “string” compounds that can be straight or branched chain. The strings are represented as HC≡CH for acetylene instead of simply C2H2. The carburizing gases thus used in vacuum furnaces prior to this invention had hydrocarbon in which the carbons were bound together in a string or chain.
The vacuum processes in the prior art discussed above have been confronted with at least two problems. The first problem is that they have only been able to supply a level of carbon at saturation or above. The high carbon potential is often rejected by many because carbide networks are typically formed which is undesirable. To combat the carbide network previous methods have removed the carbon bearing gas by evacuation or the turning off of the plasma to allow the carbide networks to diffuse away or homogenize into the steel. This approach does work, but it is not truly desirable since the carbide networks are considered bad in most cases.
As noted, when unsaturated aliphatic hydrocarbons break down during carburizing, they produce a by-product known as soot which includes tar as well as solid carbon particles. The soot collects in the furnace after the process and must be removed. This requires extra maintenance and expense to keep the operation clean and reduces productivity. The higher order hydrocarbons especially have a tendency to deposit soot. In the one article cited, high quantities of hydrogen are introduced into the furnace, which could, in theory, raise repeatability issues. In the '540 acetylene patent, high vacuum levels are required to prevent soot formations, according to the theory of that patent.
In all the vacuum technology prior art, the carburizing gas is introduced at levels sufficiently high to saturate the workpiece surface and the gas metering is stopped to allow diffusion. This results because there is no way to control the carbon potential in the vacuum environment. For gas atmosphere carburizing a CO/CO2 ratio can be maintained. However, oxygen does not exist in a vacuum carburizing process and the vacuum drawn is constantly drawing out the carburizing gas.
Insofar as vacuum carburizing apparatus is concerned, all conventional apparatus meters the carburizing medium into the furnace chamber as a gas. In fact, all industrial furnace heat treat processes (other than the salt bath class) use gas, although there are one or two known instances where kerosene was dripped into a positive pressure furnace chamber (gas carburizing) or where one of the pretreatment gas pressures was inadvertently controlled so that liquid nitrogen was inadvertently injected into the furnace. Carburizing occurs immediately upon introduction of the carburizing medium into the furnace chamber and the vacuum reduces the moles of carburizing gas present in the furnace chamber. The controllability of the process is therefore a function of the sensitivity of the mass flow controller and the ability of the gas flow metering valves to meter the gas. In vacuum carburizing (and until this invention), no in-situ measurements of the gas in the furnace chamber were taken. Only one gas was used and the gas flow was set at a carbon potential to produce a saturated iron carbide surface that was subsequently diffused into the case. Where an additional gas was used (hydrogen or nitrogen for carbonitriding), that was also set at a fixed quantity. In summary, for a number of reasons, there is not believed to be any in-situ gas control of the vacuum carburizing process until this invention.
For the higher order unsaturated aliphatic hydrocarbons which are highly reactive the system that is used to pressurize and deliver the gas to the furnace can affect the composition of the gas metered into the furnace. Depending on the purity of the feedstock and the gas delivery system, variations in the hydrocarbon make-up can occur. While the fact that there may be some cracking of the hydrocarbons in the delivery system will not materially alter the carburizing process (since the HC must be cracked anyway to produce the carbon) in theory variations are possible in the gas delivered to the furnace and this relates to precise control and repeatability of the process.
For acetylene, the complications may be more severe. Acetylene in the pressurized cylinder form is supplied with acetone as one of the components. The weight of acetone settles that component to the bottom of the cylinder. As the contents of the cylinder are consumed to provide acetylene gas, the gas layer at the top of the cylinder can, in theory, carry some acetone with it. Acetone produces oxygen on decomposition which is to be avoided in vacuum carburizing. Thus, the possibility of acetone in the acetylene gas increases as the bottled acetylene tank is used up.